There is a general requirement for coders which can provide outputs which allow decoding at a receiver selectively at least two different coding schemes. For example, a coder could transmit signals which could be decoded to provide lower or higher resolution depending on the receiving equipment. This principal is extendable to multi-layered coders.
One known way of configuring such a hybrid, or compatible, layered coder is shown at FIG. 1 in which a base layer is coded at a lower resolution with the higher resolution differences being coded by a second coder to form an enhancement layer which codes the differences (error signals) between the decoded base layer and the input signals. A receiving decoder can either decode the base layer alone to provide a lower resolution picture or obtain a higher resolution picture by further decoding the enhancement layer and adding that to the decoded base layer signals.
In such prior art coding schemes the enhancement layer coder does not carry out any predictive coding on the error signal between the base layer coded signal and the input signal.
Where the higher resolution coding scheme is to operate at high bit rates, for example 5 to 10 Mbit/s, the amount of overhead data associated with predictive coding using motion vectors, for example, is very small relative to the coded data. In these cases the picture quality is optimised by providing such predictions and efficiently coding the prediction error of the predictive coder.
One known approach to a hybrid coder in which the higher resolution coder utilizes predictive coding is shown at FIG. 2. In order to utilise the output of the lower resolution coder, the decoded output of the lower resolution coder is used as a estimate of the signal to be encoded by the higher resolution coder. This estimate is used in preference to the estimate provided by the higher resolution coder when it provides a better estimate.
In practice it has been found that if the incoming video signals to be encoded is an intraframe then the coded signal provided by the lower resolution coder does provide an estimate which can be subtracted from the incoming frame and the difference subsequently coded. However, the prediction provided by the base layer coder contains at best only the low frequencies of the incoming video frame and in practice will also include low frequency coding errors. This prediction is found to be only very rarely better than the prediction based on the output of the higher resolution coder which at the higher bit rate concerned are very near to the original pictures. The higher resolution coder therefore rarely uses the output from the lower resolution coder as a prediction and the bits allocated to these lower resolution pictures in the base layer do not, therefore, generally contribute to the full resolution output picture quality.
For applications in which intracoding may be infrequent, for example in continuous broadcasts of video-conference pictures, the prior art coding scheme of FIG. 2 will in most cases perform as if there were two independent coders each providing their respective lower and higher resolution coded pictures.
It has been shown that multi-layered coding is no less efficient than single-layer coding provided that overheads can be ignored. This assumes that a single prediction error is partially coded by each layer, in which case all the transmitted bits contribute towards the higher resolution layer's codes picture quality. This is an indication, therefore, that the prior art configuration of FIG. 2 is not operating efficiently as in most cases the lower resolution bits do not contribute to the high resolution picture quality.